In order to aid the hearing impaired, written text corresponding to spoken dialog is encoded into transmitted television signals. This text is decoded in a decoder at the viewing location of the television program, and is displayed over the video signal of the television set, whereby the audio portion of a transmitted program may be read from the television screen. The text is typically displayed adjacent the bottom of the television screen. The encoding of the text signal into the television signal whereby it is not displayed unless decoded is referred to a closed captioning.
A North American closed captioning standard was originally proposed by The Closed Captioning Association and the Public Broadcasting System (PBS). The standard is defined by the FCC in FCC document FCC-91-119 and in the FCC Federal Register Vol. 56 No 114.
The standard specifies that closed captioned data should be transmitted on scan line 21 of every odd video field of the North American interlaced video signal. The closed captioned signal is sent as a black and white (luminance only) signal with no color component. Each odd video field which contains closed captioned data is prefixed with a 0.5035 MHz sinusoidal run-in signal which is intended to be used at the decoder to lock to the closed captioned data. This run-in signal is illustrated as element 1 in FIG. 1. A start bit 2 immediately follows the run-in signal 1, which is immediately followed by 16 bits of closed captioning data 3, These bits form two 7 bit ASCII codes plus odd parity bits, forming two bytes 4 of closed captioned data. These two bytes can define text or define color, style, and attribute information. The bytes may also contain information which controls the mode of the decoder, which defines whether the text should be shown full screen, 2, 3 or 4 line roll up, paint on, and pop on. Cursor positioning data may also be defined.
In prior art closed captioned signal decoding systems a decoder is connected between the incoming video signal and the television set. When activated, the decoder decodes the closed captioned data signal and produces a video signal containing the closed captioned text, which replaces part or all of a television program which is being watched. The result is that some portion of the broadcast image is obscured by the closed captioned text. Further, passing through the decoder the image to be displayed on the television screen may be degraded.